What is the maximum altitude that the APU can be used to start a Main Engine?

• A. Flight Level 180
• B. Flight Level 210
• C. Flight Level 230
• D. Flight Level 260

Answer: (B)

The test is asking about how high the APU can provide enough bleed air to start a main engine. The APU supplies pneumatic power to the engine start system, and it must deliver sufficient pressure and flow to light off the engine. On the CRJ‑550, that capability is limited to Flight Level 210 (21,000 feet). Above this altitude, the air is too thin for the APU to maintain the necessary bleed air pressure to reliably start a main engine. So, the maximum altitude for starting a main engine with the APU is FL210.